Many prosthetic joints are designed as simple hinges allowing the tibia to swing about a fixed fulcrum on the femur. This arrangement does not duplicate the natural motion of the human knee and is therefore inconvenient in cases where most of the tibia and the soft tissue around the knee, including muscles, veins and tendons, have remained intact. Even in a full prosthesis such a joint makes walking difficult, especially where the other leg functions normally.
It has already been proposed (see, for example, Austrian Pat. No. 212,973) to connect a tibia to a femur via a pair of extensible, spring-loaded intersecting links urging a convex ridge on the femur into contact with a concave track on the tibia. Since the ligaments of the natural knee joint are practically inextensible, being maintained in a taut state by the tissue surrounding the condyles, such a system also fails to reproduce the normal knee action.
In a somewhat similar system, described in German Pat. No. 841,190, the two leg portions are interconnected by a pair of rigid links which in a limiting forward position, conforming to a stretched leg, define with the associated mountings a four-bar kinematic linkage of substantially triangular configuration, i.e. with three of their four hinge points or fulcra approximately in line with one another. This, too, represents a substantial deviation from the natural joint.